Your Ad HereColusa County is a county located in the U.S. state of California. As of 2016, its population was 21,588. The county seat is Colusa.HistoryColusa County is one of the original counties of California, created in 1850 at the time of statehood. The county was named after two Mexican land grants; Coluses (1844) and Colus (1845). The name of the county in the original state legislative act of 1850 was spelled Colusi, and often in newspapers was spelled Coluse. The word is derived from the name of a Native American tribe living on the west side of the Sacramento River. The name was changed to Colusa shortly after. This however is disputed. The actual origin of the name is unknown.